The document discusses the effects of adding strontium and silicon carbide on the mechanical properties of an aluminum alloy (LM6). Key findings include: 1) Adding 0.5 wt% strontium and 10 wt% silicon carbide improved the ultimate tensile strength and hardness of the aluminum alloy the most compared to other concentrations tested. 2) The microstructure was modified by adding strontium, changing the silicon morphology from needle-like to fibrous and increasing the amount of alpha-aluminum dendrites, leading to enhanced properties. 3) While strengths increased with the additions, the tensile strength of the composites did not increase dramatically due to weak particle-matrix interfaces. Heat

1. IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE)
e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 12, Issue 4 Ver. I (Jul. - Aug. 2015), PP 32-38
www.iosrjournals.org
DOI: 10.9790/1684-12413238 www.iosrjournals.org 32 | Page
Effect of Strontium and Silicon Carbide on Mechanical Properties
of Aluminum Alloy (Lm6)
Negin Ashrafi, Ali Hussein Humod
1
(Department of Mechanical and Manufacturing Engineering, 43400 Serdang, Malaysia)
Abstract: A study of the addition different weight percentages of strontium and its influence on the mechanical
properties and microstructure of Aluminum LM6 modified by (0.01, 0.02, 0.5) wt% strontium (Sr) was
conducted, with the purpose of evaluating the properties of particulate silicon carbide (Sic), as reinforcement
as well as the effect of a high Sr content. The obtained results indicated that addition of Al-10Sr improves the
UTS and hardness of Aluminum and composite. Moreover, it is observed that adding Sr to the Al–SiC composite
increased the amount of α-Al. The mechanical properties of the composite can be related to the volume fraction
of dendritic α-Al, which can improve the mechanical properties of Aluminum or Al-Sic composite. it is
concluded that the tensile for the composite did not increase dramatically because of weak interface between
particles and matrix decrease the UTS, on the other hand strong interface between particles or fibers in the
matrix show high stiffness and strength but typically a low resistance to fracture. Before tensile testing, heat
treatment should apply to the samples to increase the ductility and ultimately UTS. In this research, a scanning
electron microscope equipped with EDS is used to define the modification effects on microstructure and as
result, on mechanical properties.
Keywords- AL-SiC composite; Stir casting; Sand casting; Modifier; LM6.
I. Introduction
Aluminum matrix composites reinforced with silicon carbide particles are expected to find many
applications in the automobile and electronic industries. In addition, it was found that Al-SiC composite
materials have a distinctive set of material characterizations, which are appropriate for all electronic industrial
usage that requires thermal management, It is often used for Advanced Microelectronic Packaging [1,6].
Furthermore, it is included that reinforcing Aluminum alloys with discontinuous second-phase particles offers
high strength, high modulus, greater wear resistance and desirable thermal expansion. Nowadays, Aluminum-
Silicon alloys are typically used in the electronic and different industries due to their good casting ability,
lightweight, brilliant corrosion resistance, and lower coefficient of thermal expansion. Although in recent years,
the development of lightweight aluminum alloys has improved, their properties still cannot encounter with the
requirements of productive application. Hence, it is well known Strontium as an alloy modifier is a usefull
method of refining grain size microstructure and reducing LM6 porosity of improving formability for
composites. Strontium is reportedly effective and can positively influence porosity formation, but has lower
effect on reducing the amount of visible porosity on samples [2,3].
It has been reported by some researchers that multi-axis surface stiffness in MMCs reduces the ductility
of the Aluminum matrix. There is an optimum amount of added particles to fabricate composites by increasing
10 to 15% SiC in aluminum. Furthermore by adding strontium it should be mentioned that the tensile properties
of the composite has to be increased [4]. Effective reinforcement needs good bonding of the filler and matrix,
principally for short fibers. Chemical bonding, inter-diffusion, van der Waals bonding, and mechanical joining
are interface mechanisms referred to as filler–matrix bonding. The concentration of chemical bonds may be
enhanced in several ways: (i) chemical treatment of the filler, (ii) consuming an appropriate sizing (coating) on
the filler particles or fibers, and (iii) applying a molecular coupling agent. Among the consequences of weak
interfaces is lower stiffness, hardness and strength but higher resistance to fracture. On the other hand, a strong
interface between particles or fibers in the matrix exhibits high stiffness and strength but typically low resistance
to fracture, i.e., the features of the interface affect the brittle behavior, resistance to creep, and fatigue [5,10].
Good wettability between matrix and particles also increases the tensile strength of composites [8].
II. Experimental
The materials in this experiment are defined below:
Aluminium-11.8% silicon alloy (LM6) is used as the matrix for the MMC. The chemical composition
of the alloy is given in Table1. LM6 alloy is basically a hypoeutectic Al-Si alloy (naturally containing 11.5 wt%
Si and less than 12.6 wt% Si of the eutectic composition) with little copper content (<0.1 wt%).
LM6 (aluminum casting alloy) was melted at 700°C in an electrical induction furnace. The exact
amount of Aluminum can be defined based on the capacity of each pattern and mold. It takes around 1 hour to

2. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 33 | Page
completely melt LM6 inside the furnace. after melting temperature reached 740 ℃ roughly, the Al-10Sr alloys
(0.01%, 0.02% and 0.5%), were added to the melt.
It is important to note that Strontium was not pure in this experiment. For the first and second
experiments, the Strontium was held for 10 min, and for the third experiment, it was held for 15 min in the melt.
After that the melt was poured into a a ladle and then into a sand mold. The sand consisted of silica sand,
sodium silicate, calcium and water, which was mixed in a mixer. The pouring temperature was below 650°C but
still above the melting temperature [7].
Table 1.The chemical composition of LM6
Cu Mg Si Fe Mn Ni Zn Lead Tin Others Al
0.1 0.1 1 0.6 0.5 0.1 0.1 0.1 0.05 0.2 85.95
II.1 composite prepration
The LM6 aluminum casting alloy was melted at 750°C in the electrical induction furnace. 10 wt%
Silicon carbide was preheated at 800°C for 1 hour to eliminate moisture and gas from the surface of the
particulates. The Sic powder was poured into metal cups, in which it was kept for 1 hour inside the muffle
furnace. The muffle furnace is JELRUS Temp-Master brand with maximum temperature of 1000°C. In this
research, 800°C was selected for 1 hour. Subsequently, the furnace was opened carefully; the particles were
poured into the melted Aluminum and mixed.
The main challenge with MMC fabrication is to acquire homogenous dispersion of the ceramic
particles by applying low-cost equipment for commercial applications. A stir casting setup consisting of an
induction furnace and stainless steel mixer assembly was employed to synthesize the metal matrix composite.
The vortex method is a cost-effective approach used for particle distribution in molten metal, as the particles
mix homogenously through the matrix.
III. Result And Discussion
III. 1 Hardness Test
The central part of each sample was prepared. The surface of each one was polished to eliminate
coarseness in preparation for hardness testing. The hardness was tested with a Rockwell Hardness tester. The
device used for this experiment is MITITOYO ATK-600. The ball indenter was 1/16 inches in diameter and the
total load selected was 15 kgf. The tests were carried out on 5 different, random areas on each sample surface,
and then the average was calculated for each sample. The Rockwell hardness number was obtained by
calculating the average of the obtained data is shown in Fig 1. The best result is obtained by adding 0.5 wt% Al-
10Sr and 10 wt % Sic to Aluminum.In Table 2 Hardness of Al by various amount of Sr and Sic is given. The
mixer assembly contained a stirrer, which was attached to an adjustable speed vertical motor of maximum 400
rpm by means of a shaft. After mixing 10% Sic with Aluminum, 0.02 and 0.5 wt% Sr was added to the molten
LM6 inside the furnace. Strontium remained in the molten Aluminum for 10 and 15 min respectively. The speed
of the stirrer was slowly raised from 100 to 400 rpm and the preheated reinforced particles were added to the
molten metal. After adding the reinforcement, the materials were continuously stirred for 3-4 minutes so the
prepared particles would mix adequately in the matrix.
Fig. 1 Hardness test result by Rockwell

3. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 34 | Page
Table 2 Hardness of Al by various amount of Sr
III.2 Ultimate Tensile Testing Strength
Tensile testing was done with INSTRON 3382 equipped with a digital controller. The dimensions of
each sample should be defined before testing and then be input into the device. All samples were labeled and
their widths and thicknesses measured. Software was connected to the device to measure the stress and strain
data until the samples fractured. The speed used for this experiment was 1 mm/min. Although UTS prediction
can be very complicated in some cases when producing composites. Figure 2 shows that the highest UTS was
for Al with 0.5% Sr [11]. It is observed that the composite’s UTS not improved. The strong interface between
particles or fibers in the matrix shows high stiffness and strength but typically low resistance to fracture
Fig 2. UTS with various amount of Sr, SiC
Table 3.UTS with various amount of Sr,SiC
Number Sample UTS (Mpa)
1 Al-0.01 % Sr 153.09
2 Al-0.02 % Sr 154.165
3 Al-0.5% Sr 175.96
4 Al-10% Sic+0.02Sr 163.15
5 Al-10% Sic+0.5 Sr 167.94
III.3 Microstructure Analysis
Samples of the metal matrix composite and aluminum LM6 were prepared for metallographic
examination. A sample size of 10x10 mm was cut from each section. Abrasive paper of 320, 400, 600, 800, and
1200 grit was used for sample preparation using a machine. The samples were then polished using the same
machine. For grinding, as well as coarse polishing and finally fine polishing, Buehler suspension with 0.05
micron fineness was used. The available machine was Hitachi S-3400. The microstructure of casted LM6 was
tested at different locations on the sample surfaces.
The eutectic phase of unmodified Al-Si alloy contains a silicon phase with rod-angular and flake-like
morphology, which causes poor ductility and brittleness. As mentioned before, modification was done to change
the form of the silicon phase from flaky to fibrous.
Generally, modification can be attained by quick solidification (quench solidification) or chemical
modification. Small quantities of strontium can transform the morphology of the eutectic silicon phase existing
in Al–Si casting alloys from coarse plate-like to fine fibrous networks.

4. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 35 | Page
It is observed that adding Sr to Al–SiC composite increases the amount of α-Al. The mechanical
properties of the composite are related to the volume fraction of dendritic α-Al, which can improve the
mechanical properties of Aluminum or Al-Sic composite [9]. Al4C3 was also produced at the interface between
the Sic and Aluminum matrix. Figure 3(a) shows the composite microstructure consisting of three main phases:
Aluminum, eutectic phase-Si and Al4C3. The Al4C3 composition is not desirable for several reasons. It clearly
damages reinforcement, and Al4C3 may also be at risk of corrosion and it could be accompanied by a rise in Si
levels. An alternative method of restricting the reaction is surface treatment, which would also favorably
improve wettability and ductility.
a)
a-1)

5. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 36 | Page
b)
b-1)
C
D
Fig 3. a,a-1) Al-10wt % Sic-0.5wt % Sr, b,b-2) Al-10wt % Sic-0.02wt % Sr , c)Al-0.02wt % Sr, d) Al-0.5% Sr

6. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 37 | Page
III.4 EDS (Energy Dispersive Spectroscopy)
In Figure 4 it is shown that the Sr component is obviously not detected among the other chemical
components .The spectrum of Sr-modified LM6 is similar for samples of Aluminum modified with different
amounts of Sr due to the similar amounts of chemical present. In Tables 4, the oxygen content shows significant
variation; however, the weight percentage of Al was decreased compare to LM6. on the other hand, the Al-
0.02% Sr has a highest amount of silicon detected by EDS on the surface. it is included that this may because of
higher silicon was precipitated in the eutectic Si phase because of more area of eutectic solidification.
Fig.4 Chemical composition Al-0.01%Sr
Table 4. Quantitative of Chemical composition
Element
Line
Al-0.01%Sr Al-0.02% Sr Al-0.5% Sr Al-0.02%Sr-
10%SiC
Al,LM6
Weight % Weight % Weight % Weight % Weight %
C K 7.43 7.33 6.68 6.31 4.16
O K 13.30 2.90 7.33 5.47 3.24
Al K 69.52 74.20 75.66 75.21 82.05
Si K 9.75 15.56 9.60 13.00 9.40
Si L --- --- --- --- ---
Total 100.00 100.00 100.00 100.00 100.00
IV. Conclusion
The following conclusions can be drawn from the experimental results:
 The microstructure of the Aluminum has been changed dramatically by adding Al-10Sr. By increasing the
amount of Al-10 Sr white blobs of primary alpha dendrites, surrounded by a fine, fibrous eutectic mixture
of alpha and Si increased. Modification treatment altered the Silicon morphology from a needle-like
structure to a fibrous structure, which enhanced the mechanical properties.
 The best result for hardness is obtained by adding 0.5 wt% Al-10Sr and 10 wt % Sic to Aluminum by
vortex method. The microstructure modification can be reason which effect on improving the hardness of
composite. Addition of Al-10Sr improves the hardness of Aluminum. The maximum amount of hardness is
gained by adding 0.5 wt % of Al-10Sr modifier

7. Effect of Strontium and Silicon Carbide on Mechanical Properties of Aluminum Alloy (Lm6)
DOI: 10.9790/1684-12413238 www.iosrjournals.org 38 | Page
 The UTS of Aluminum increased by adding SiC and Sr as well as hardness. the Maximum amount was
175.96 for LM6. It is evident that the composite’s tensile strength did not increase dramatically. Thus, it is
concluded that the weak interface between the particles and matrix decreased the UTS. Nonetheless, the
strong interface between particles or fibers in the matrix demonstrated high stiffness and strength but low
resistance to fracture. Prior to tensile testing, heat treatment should be applied to the samples to increase
ductility and ultimately UTS.
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